초록
A three-parameter logistic growth model was modified to monitor the glucose uptake profile of yeast during very-high-gravity (VHG) ethanol fermentation. The modified model was used to define t<SUB>50</SUB> as a quantifier to differentiate among various fermentation conditions. There are two types of t<SUB>50</SUB>; t<SUB>50</SUB><SUP>g</SUP> is the time required to convert 50% of the initial glucose, and t<SUB>50</SUB><SUP>e</SUP> is the time required to produce half of the final ethanol. A 2<SUP>4</SUP> factorial experimental design was implemented to illustrate the applicability of using t<SUB>50</SUB> to isolate active ingredients in VHG growth media. The analytical results obtained from the experimental design and from a modified model were compared, which demonstrated that t<SUB>50</SUB> could serve the proposed objectives. A shorter t<SUB>50</SUB> implies a faster fermentation. A tailing of the ethanol profile after t<SUB>50</SUB><SUP>e</SUP> indicates that there is an inhibitory effect imposed on yeast, i.e., the stronger the tailing in the ethanol profile, the stronger the inhibitory effect. When t<SUB>50</SUB> is equal to or near to the halftime of the total course of the fermentation, a bell-shaped curve was seen for the glucose uptake rate or for the ethanol production rate, indicating that the inhibitory effect exerted on yeast was evenly distributed.